In mobile telecommunications, the term handover or handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. There may be different reasons why a handover might be required in mobile telecommunications networks, among which re for example,
when the mobile device is moving away from the area covered by one cell and entering the area covered by another cell, in which case the call is transferred to the second cell in order to avoid call termination when the mobile device leaves the range of the first cell;
when the capacity for connecting new calls of a given cell is used up (i.e. the cell becomes overloaded) and an existing or new call from a mobile device, which is located in an area overlapped by another cell, is transferred to that cell in order to free-up some capacity in the first cell for other users, who can only be connected to that cell;
in non-CDMA networks when the channel used by the mobile device becomes interfered by another mobile device using the same channel in a different cell, the call is transferred to a different channel in the same cell or to a different channel in another cell in order to avoid the interference;
when the user behavior changes in non-CDMA networks, e.g. when a fast-travelling user, connected to a large, umbrella-type of cell, stops then the call may be transferred to a smaller macro cell or even to a micro cell in order to free capacity on the umbrella cell for other fast-traveling users and to reduce the potential interference to other cells or users;
The most basic form of handover is when a communication session, to which the mobile device is part, is redirected from its current cell (referred to as “source cell”) to a new cell (referred to as “target cell”). The purpose of such handover is to maintain the call as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell.
For practical implementations of handoffs in a cellular network, each cell is assigned with a list of potential target cells, which can be used for handing-off calls from this source cell to these potential target cells, called neighbors whereas the list is called neighbor list. Creating such a list for a given cell is not trivial and different algorithms may be applied to input data retrieved from field measurements or computer predictions of radio wave propagation in the areas covered by the cells.
Typically, during a communication session one or more parameters of the signal in the channel in the source cell are monitored and assessed in order to decide when a handover may be required. The downlink (forward link) and/or uplink (reverse link) directions may be monitored. The handover may be initiated by the mobile device or by the base station (BTS)/by an E-UTRAN Node B (“eNB”) in the LTE Standard, of its source cell and, in some systems, by a BTS of a neighboring cell. The mobile device and the BTSs of the neighboring cells monitor each other signals and the best target candidates are selected by the BS of the source cell from among the neighboring cells.
This need to hand over mobile devices from one cell to another has been increased in the recent years as wireless networks operators have started to deploy their own or rely on end users to buy very small Base Stations, in order to meet the increasing demand for data traffic. This new type of cell sites, referred to herein below as “small cells” or “metro cells”, used in conjunction with wireless cells of the traditional cellular networks (macro cells). Networks that include both macro cells and metrocells are referred to herein as heterogeneous networks (HetNets).
The term “small cells” as used herein and throughout the specification and claims encompass femtocells, picocells microcells and metrocells. Small-cell networks can also be realized by means of distributed radio technology consisting of centralized baseband units and remote radio heads. A common factor in all these approaches to small cells is that they are centrally managed by mobile network operators.
In analog systems, the parameters used as criteria for requesting a handover are usually the Received Signal Strength Indicator (“RSSI”) and the received Signal-to-noise ratio (“SNR”). In non-CDMA 2G digital systems the criteria for requesting handover may be based on estimates of the received signal power, bit error rate (BER) and block error/erasure rate (BLER), received quality of speech (RxQual), distance between the mobile device and the BTS (estimated from the radio signal propagation delay) and others. In CDMA systems, 2G and 3G, the most common criterion for requesting a handover is Ec/Io ratio measured in the pilot channel (CPICH) and/or RSCP.
Section 8.3.8 of ETSI's “LTE: Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) (3GPP TS 36.423 version 9.4.0 release 9)” relates to mobility setting change which enables an eNB to negotiate the handover trigger settings with a peer eNB controlling neighboring cell, or in other words, the goal of the procedure is to renegotiate the mobility settings of target cell eNB2 when offloading UEs from the source cell, eNB1, to the target cell, eNB2. The procedure uses non UE-associated signaling, in which eNB1 sends a MOBILITY CHANGE REQUEST to its neighbor eNB2 and if that request is accepted by eNB2, the latter responds by returning a MOBILITY CHANGE ACKNOWLEDGE to eNB1. An example of such a process is illustrated in FIG. 1.
However, there is still an inherent problem associated with such a method for carrying out the handoff procedure, when some of the mobile devices which have just been handed over from eNB1 to eNB2 are immediately thereafter pushed back by the eNB2 to eNB1, and vice versa.
According to above-mentioned procedure, the eNB of the overloaded cell determines that it needs to handover some of the mobile devices, which it serves to its neighbors in order to reduce its load. As illustrated in FIG. 1, the eNB sends a MOBILITY CHANGE REQUEST to one or more of its neighbors identifying a change that is required in the neighbors' operating conditions, a change which will enable handing over the mobile devices as determined. Each of the neighbors that are approached by the eNB sends either a confirmation or a rejection to that request. If the eNB receives a rejection it will re-negotiate with that neighbor a different change (a smaller change), which hopefully will be acceptable to that neighbor and still will ease the situation for the originating eNB. This is a complex and resource consuming process, which still does not necessarily result in an optimized operation. There are several possible reasons for this drawback, for example:                Fast connection (e.g. X2 link) does not necessarily exist between neighbor eNBs; and        The process performance strongly depends on particular details of the load balancing algorithms, which are implemented at both eNB1 and eNB2. When both eNBs are produced by the same vendor, these details are synchronized with each other, however this not the case if they are both produced by different vendors.        